The usual procedure for the preparation of azo dyes is to diazotise the amine employed as diazo component in a first step and then, in a second step, to react the diazotised amine with the appropriate coupling component. The diazotisation is normally carried out in a mineral acid solution by adding an excess of nitrite, e.g. sodium nitrite. When diazotisation is complete, the excess nitrite must be removed before the azo coupling takes place. This removal is usually effected by adding a small amount of non-diazotised amine or by adding urea or amidosulfonic acid (sulfamic acid). For diazotisation on an industrial scale the rapidly acting sulfamic acid is normally added. However, the drawback of this procedure is that if an excess of sulfamic acid is added, especially where diazonium salts containing strongly negative substituents are obtained, it may result in secondary reactions, e.g. in the reaction of the sulfamic acid with the diazonium salt so that the amine is formed again by rediazotisation (q.v. H. R. Schweizer, Kunstliche Organische Farbstoffe und ihre Zwischenprodukte, Springer Verlag 1964, page 182). To avoid this yield-diminishing secondary reaction it is necessary to add the sulfamic acid as accurately as possible in an amount just sufficient to destroy the excess nitrite. Especially in continuous reactions, monitoring the concentration of nitrite in the diazotisation solution by the widely employed spot test on potassium iodide starch paper has not proved at all suitable Hence it is the object of the present invention to provide a method that permits continuous monitoring of the nitrite concentration and a smooth controlled addition of agent for removing nitrite.